Coextruded, hot-sealable and peelable polyester film having high peeling resistance, process for its production and its use

ABSTRACT

This invention relates to a coextruded, transparent, biaxially oriented polyester film comprising a base layer (B) and a heatsealable top layer (A) which is peelable from at least the APET side of an APET/CPET tray, the heatsealable and peelable top layer (A) consisting of
     a) 50–95% by weight of polyester and   b) 5–40% by weight of a polyester-incompatible polymer (=anti-PET polymer)
 
based on the mass of the top layer (A), wherein
   c) the polyester is composed of 20–95 mol % of units derived from at least one aromatic dicarboxylic acid and 5–80 mol % of units derived from at least one aliphatic dicarboxylic acid, and   d) the layer thickness d A  is from 1.3 to 3.0 μm.   

     The invention further relates to a process for producing the film and to its use.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.10/645,122 filed Aug. 21, 2003, abandoned hereby incorporated byreference herein in it's entirety. This application further claimspriority through its parent application to German Patent Application 10318 102.4, filed Apr. 22, 2003, also hereby incorporated by referenceherein in its entirety.

FIELD OF THE INVENTION

The invention relates to a coextruded, peelable, transparent andbiaxially oriented polyester film having a base layer (B) and at leastone top layer (A) applied to this base layer (B). The top layer (A) isheatsealable and features medium to strong, robust peelability, inparticular to the APET side of APET/CPET trays (APET=amorphouspolyethylene terephthalate (PET); CPET=crystalline PET). Theheatsealable and peelable top layer (A) comprises polyester based onaromatic and aliphatic acids and aliphatic diols. In addition, the toplayer (A) comprises a polyester-incompatible polymer in a certainconcentration. The invention further relates to a process for producingthe film and to its use.

BACKGROUND OF THE INVENTION

For ready-prepared meals, there are currently double-figure growth ratesin Europe. The ready-prepared meals are transferred to trays after theirpreparation (cf. FIG. 1). A film which is heatsealed to the edge of thetray seals the packaging and protects the ready-prepared meal fromexternal influences. The ready-prepared meals are suitable, for example,for heating in a microwave, for heating in a conventional oven or forheating in a microwave and in a conventional oven. In the latter case,the ready-prepared meal and the packaging have to be “dual ovenable”(=suitable for microwave and conventional ovens). As a consequence ofthe temperatures existing in the conventional oven (up to 220° C.),particularly high demands are made on the packaging material (tray andlid film).

Both for the tray and for the lid film, only selected materials can beconsidered for dual ovenable applications. Typical materials for thetrays are in this case CPET, aluminum, cardboard coated with PET or withPET film or APET/CPET trays. APET/CPET trays (cf. FIG. 1) consistexternally of a CPET layer and internally (=facing toward theready-prepared meal) of an APET layer. The thick crystalline CPET layerwhich is usually pigmented, i.e. filled with particles, provides thestability of the tray, even at the comparatively high temperatures inthe conventional oven. In contrast, the amorphous PET essentiallyimproves the adhesion of the film to the tray.

In dual ovenable applications, the material used for the lid film isgenerally PET which is sufficiently dimensionally stable and solid evenat 220° C. Materials such as PP or PE are ruled out from the outsetbecause of their low melting points. The requirements on the lid filmare best fulfilled by biaxially oriented polyester film.

When preparing the ready-prepared meal in the oven, the polyester filmis removed by hand from the tray shortly before heating or shortly afterheating. When this is done, the polyester film must on no account startto tear, start and continue to tear or tear off. The removal of the filmfrom the tray without the film starting or continuing to tear or tearingoff is also referred to in the foods industry as peeling. For thisapplication, the polyester film therefore has to be not onlyheatsealable, but in particular also peelable. For a given material andgiven overall thickness of the film, the peelability of the film isdetermined mainly by the properties of the surface layer of the filmwhich is sealed to the tray.

The peelability of films can be determined relatively simply in thelaboratory using a tensile strain tester (for example Zwick) (cf. FIG.2). For this test, two strips of breadth 15 mm and length approx. 50 mmare first cut out of the polyester film and the tray and sealedtogether. The sealed strips are, as shown in FIG. 2, clamped into theclips of the tester. The “angle” between the film clamped in the upperclip and the tray strip is 180°. In this test, the clips of the testerare moved apart at a speed of 200 mm/min, and in the most favorablecase, the film is fully removed from the tray (based on ASTM-D 3330).

In this test, a distinction is to be drawn between essentially twodifferent mechanisms.

In the first case, the tensile force rises rapidly in the course of thepulling procedure up to a maximum (cf. FIG. 3 a) and then falls directlyback to zero. When the maximum force is attained, the film starts totear, or before delamination from the tray, tears off, resulting in theforce falling immediately back to zero. The film is in this case notpeelable, since it is destroyed. The behavior of the film can rather bedescribed as a kind of “welding” to the tray. The destruction of thefilm on removal from the tray is undesired, because this complicates theeasy opening of the packaging without tools such as scissors or knives.

In contrast, a peelable film is obtained when the tensile force or thepeeling force rises up to a certain value (i.e. up to a certain plateau)and then remains approximately constant over the distance over which thetwo strips are sealed together (cf. FIG. 3 b). In this case, the filmdoes not start to tear, but rather can be peeled off as desired from thetray with a low force input.

The size of the peeling force is determined primarily by the polymersused in the sealing layer (A) (cf. FIG. 4, polymer 1 and polymer 2). Inaddition, the size of the peeling force is dependent in particular onthe heatsealing temperature employed. The peeling force generally riseswith the heatsealing temperature. With increasing heatsealingtemperature, the risk increases that the sealing layer might lose itspeelability. In other words, a film which is peelable when a lowheatsealing temperature is employed loses this property when asufficiently high heatsealing temperature is employed. This behavior isto be expected in particular in the case of polymers which exhibit thecharacteristics shown in FIG. 4 for polymer 1. This behavior which tendsto generally occur but is rather unfavorable for the application has tobe taken into account when designing the sealing layer. It has to bepossible to heatseal the film in a sufficiently large temperature rangewithout the desired peelability being lost (cf. polymer 2 in FIG. 4). Inpractice, this temperature range is generally from 150 to 220° C.,preferably from 150 to 200° C. and more preferably from 150 to 190° C.

The heatsealable and peelable layer is applied to the polyester film inaccordance with the prior art, generally by means of offline methods(i.e. in an additional process step following the film production). Thismethod initially produces a “standard polyester film” by a customaryprocess. The polyester film produced in this way is then coated in afurther processing step in a coating unit offline with a heatsealableand peelable layer. In this process, the heatsealable and peelablepolymer is initially dissolved in an organic solvent. The final solutionis then applied to the film by a suitable application process(knifecoater, patterned roller, die). In a downstream drying oven, thesolvent is evaporated and the peelable polymer remains on the film as asolid layer.

Such an offline application of the sealing layer is comparativelyexpensive for several reasons. First, the film has to be coated in aseparate step in a special apparatus. Second, the evaporated solvent hasto be condensed again and recycled, in order thus to minimize pollutionof the environment via the waste air. Third, complicated control isrequired to ensure that the residual solvent content in the coating isvery low.

Moreover, in an economic process, the solvent can never be completelyremoved from the coating during the drying, in particular because thedrying procedure cannot be of unlimited duration. Traces of the solventremaining in the coating subsequently migrate via the film disposed onthe tray into the foods where they can distort the taste or even damagethe health of the consumer.

Various peelable, heatsealable polyester films which have been producedoffline are offered on the market. The polyester films differ in theirstructure and in the composition of the top layer (A). Depending ontheir (peeling) properties, they have different applications. It iscustomary, for example, to divide the films from the applicationviewpoint into films having easy peelability (easy peel), having mediumpeelability (medium peel) and having strong, robust peelability (strongpeel). The essential quantifiable distinguishing feature between thesefilms is the size of the particular peeling force according to FIG. 3 b.A division is carried out at this point as follows:

Easy peelability Peeling force in the range of (easy peel) from about 1to 4 N per 15 mm of strip breadth Medium peelability Peeling force inthe range (medium peel) from about 3 to 8 N per 15 mm of strip breadthStrong, robust peelability Peeling force in the range of (strong peel)more than 5 N per 15 mm of strip breadthSome sealable PET films are already known.

EP-A-0 035 835 describes a coextruded sealable polyester film to whichparticles whose average particle size exceeds the layer thickness of thesealing layer are added in the sealing layer to improve the winding andprocessing performance. The polymer of the sealing film layer issubstantially a polyester copolymer which is based on aromaticdicarboxylic acids and also aliphatic diols. The particulate additivesform surface elevations which prevent undesired blocking and adhesion ofthe film to rolls or guides. The selection of particles having adiameter greater than the sealing layer worsens the sealing performanceof the film. No information is given in the document on the sealingtemperature range of the film. The seal seam strength is measured at140° C. and is in the range from 63 to 120 N/m (corresponding to from0.97 to 1.8 N/15 mm of film breadth). There are no indications in thedocument concerning the peeling performance of the film with respect totrays made of APET, CPET and APET/CPET.

EP-A-0 379 190 describes a coextruded, biaxially oriented polyester filmwhich comprises a carrier film layer made of polyester and at least onesealing film layer made of a polyester composition. The sealing filmlayer may comprise aliphatic and aromatic dicarboxylic acids and alsoaliphatic diols. The polymer for the sealing film layer comprises twodifferent polyesters A and B, of which at least one (polyester B)contains aliphatic dicarboxylic acids and/or aliphatic diols. Thesealing energy which is measured between two sealing film layers facingeach other and joined together (=fin sealing) is more than 400g_(force)·cm/15 mm (=more than 4 N·cm/15 mm), and the sealing film layermay comprise inorganic and/or organic fine particles which are insolublein the polyester, in which case the fine particles are present in anamount of from 0.1 to 5% by weight, based on the total weight of thesealing film layer. In the examples of EP-A 0 379 190, organicparticles, when they are used at all, are used in maximum amounts of0.3% by weight. Although the film features good peeling properties(having plateau character in the peeling diagram [see above]) withrespect to itself (i.e. sealing film layer with respect to sealing filmlayer), there is no information about the peeling performance withrespect to trays made of APET, CPET and APET/CPET. In particular, thefilm of this invention is in need of improvement in its producibilityand its processibility (the raw materials tend to adhere).

WO-A-96/19333 describes a process for producing peelable films, in whichthe heatsealable, peelable layer is applied inline to the polyesterfilm. In the process, comparatively small amounts of organic solventsare used. The heatsealable, peelable layer comprises a copolyester forwhich a) from 40 to 90 mol % of an aromatic dicarboxylic acid, b) from10 to 60 mol % of an aliphatic dicarboxylic acid, c) from 0.1 to 10 mol% of a dicarboxylic acid containing a free acid group or a salt thereof,d) from 40 to 90 mol % of a glycol containing from 2 to 12 carbon atomsand e) from 10 to 60 mol % of a polyalkyldiol for forming thecopolyester were used. The coating is applied to the film from anaqueous dispersion or a solution which contains up to 10% by weight oforganic solvent. The process is restricted with regard to the polymerswhich can be used and the layer thicknesses which can be achieved forthe heatsealable, peelable layer. The maximum achievable layer thicknessis specified as 0.5 μm. The maximum seal seam strength is low, and isfrom 500 to 600 g/25 mm², or [(from 500 to 600)/170] N/15 mm of filmbreadth.

WO 02/059186 A1 describes a process for producing peelable films, inwhich the heatsealable, peelable layer is likewise applied inline to thepolyester film. In this case, melt-coating is employed, and it ispreferably the longitudinally stretched film which is coated with theheatsealable, peelable polymer. The heatsealable, peelable polymercontains polyesters based on aromatic and aliphatic acids, and alsobased on aliphatic diols. The copolymers disclosed in the examples haveglass transition temperatures of below −10° C.; such copolyesters aretoo soft, which is why they cannot be oriented in customary rollstretching methods (adhesion to the rolls). The thickness of theheatsealable, peelable layer is less than 8 μm. In WO 02/059186 A1, themelt-coating known per se is delimited from the extrusion coating knownper se technically and by the viscosity of the melt. A disadvantage ofthe process is that only comparatively fluid polymers (maximum 50Pa*sec) having a low molecular weight can be used. This results indisadvantageous peeling properties of the film. Moreover, the coatingrate in this process is limited, which makes the production processuneconomic. With regard to quality, faults are observed in the opticalproperties of the film which are visible, for example, as coatingstreaks. In this process, it is also difficult to obtain a uniformthickness of the sealing layer over the web breadth of the film, whichin turn leads to nonuniform peeling characteristics.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a coextruded,heatsealable and peelable, biaxially oriented polyester film whichfeatures outstanding peeling properties with respect to trays, inparticular with respect to the APET side of trays made of APET/CPET. Itshould no longer have the disadvantages of the prior art films andshould in particular have the following features:

-   -   A medium to strong, robust peelability (medium peel to strong        peel) with respect to the APET side of trays made of APET/CPET.        The peeling force should be in the range from 3 to 10 N per 15        mm preferably in the range from 4.0 to 10 N per 15 mm and more        preferably in the range from 4.5 to 10 N per 15 mm, of film        strip breadth.    -   No organic solvent residues are present in the heatsealable and        peelable layer.    -   The heatsealable and peelable layer, with respect to the APET        side of APET/CPET trays, has a minimum sealing temperature of        not more than 150° C., preferably not more than 140° C., in        particular not more than 130° C., and a maximum sealing        temperature of generally 220° C., preferably 200° C. and more        preferably 190° C.    -   It is produced employing processes in which no organic solvents        are used from the outset.    -   The film can be prepared economically. This also means, for        example, that stretching processes which are customary in the        industry can be used to produce the film. In addition, it should        be possible to produce the film at machine speeds of up to 500        m/min which are customary today.    -   Good adhesion (greater than 2 N/15 mm of film breadth) between        the individual layers of the film is ensured for their practical        employment.    -   The optical properties of the film are good. This means, for        example, low opacity (less than 20%) and high gloss (greater        than 70 for the sealable side and >100 for the side opposite the        sealable side; each at 20° angle of incidence) of the film.    -   In the course of the production of the film, it is guaranteed        that the regrind can be fed back to the extrusion in an amount        of up to 60% by weight, without significantly adversely        affecting the physical (the tensile strain at break of the film        in both directions should not decrease by more than 10%), but in        particular the optical, properties of the film.

In addition, care should be taken that the film can be processed onhigh-speed machines. On the other hand, the known properties whichdistinguish polyester films should at the same time not deteriorate.These include, for example, the mechanical (the modulus of elasticity ofthe biaxially stretched films in both orientation directions should begreater than 3000 N/mm², preferably greater than 3500 N/mm² and morepreferably greater than 4000 N/mm²) and the thermal properties (theshrinking of the biaxially stretched films in both orientationdirections should not be greater than 3%, preferably not greater than2.8% and more preferably not greater than 2.5%), the winding performanceand the processibility of the film, in particular in the printing,laminating or in the coating of the film with metallic or ceramicmaterials.

In this context, heatsealable refers to the property of a coextruded,multilayer polyester film which comprises at least one base layer (B)and also comprises at least one layer (=heatsealable top layer) of beingable to be bonded by means of sealing jaws by applying heat (140 to 220°C.) and pressure (2 to 5 bar) within a certain time (0.2 to 2 sec) toitself (fin sealing), or to a substrate made of a thermoplastic (=labsealing, in this case in particular the APET side of APET/CPET trays),without the carrier layer (=base layer) itself becoming plastic. Inorder to achieve this, the polymer of the sealing layer generally has adistinctly lower melting point than the polymer of the base layer. Whenthe polymer used for the base layer is, for example, polyethyleneterephthalate having a melting point of 254° C., the melting point ofthe heatsealable layer is generally less than 230° C., in the presentcase preferably less than 210° C. and more preferably less than 190° C.

In this context, peelable refers to the property of a coextrudedpolyester film which comprises at least one layer (=heatsealable andpeelable top layer), after heatsealing to a substrate (in this casesubstantially the APET side of an APET/CPET tray), of being able to bepulled from the substrate in such a way that the film neither starts totear nor tears off. The bond of heatsealable film and substrate breaksin the seam between the heatsealed layer and substrate surface when thefilm is removed from the substrate (cf. also Ahlhaus, O.E.: Verpackungmit Kunststoffen [Packing with plastics], Carl Hanser Verlag, p. 271,1997, ISBN 3-446-17711-6). When removing the film heatsealed to a teststrip of the substrate in a tensile strain testing instrument at apeeling angle of 180 in accordance with FIG. 2, the tensile strainbehavior of the film according to FIG. 3 b is then obtained. Whenpeeling off the film from the substrate commences, the force requiredfor this purpose rises, according to FIG. 3 b, up to a certain value(e.g. 4 N/15 mm) and then remains approximately constant over the entirepeeling process, but is subject to larger or smaller variations (approx.+/−20%)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary sealed tray;

FIG. 2 is a schematic illustration of a tensile strain measuringtechnique;

FIG. 3 a is an exemplary diagram of tensile strain at break for a filmhaving weldable behavior;

FIG. 3 b is an exemplary diagram of tensile strain at break for a filmhaving peelable behavior;

FIG. 4 is an exemplary diagram of tensile strain at break for a filmhaving weldable and peelable behavior;

FIG. 5 is an exemplary diagram of the correlation between sealingtemperature and peeling force.

DETAILED DESCRIPTION OF THE INVENTION

This object is achieved by providing a coextruded, transparent,biaxially oriented polyester film comprising a base layer (B) and aheatsealable top layer (A) which is peelable from at least the APET sideof an APET/CPET tray, the heatsealable and peelable top layer (A)consisting of

-   a) 50–95% by weight of polyester and-   b) 5–40% by weight of a polyester-incompatible polymer (=anti-PET    polymer)    based on the mass of the top layer (A), wherein-   c) the polyester being composed of 20–95 mol % of units which derive    from at least one aromatic dicarboxylic acid and 5–80 mol % of units    which derive from at least one aliphatic dicarboxylic acid, and-   d) the layer thickness d_(A) being from 1.3 to 3.0 μm.

The material of the top layer (A) thus consists predominantly of apolyester and a polyester-incompatible polymer. The polyester iscomposed of units which are derived from aromatic and aliphaticdicarboxylic acids. The units which derive from the aromaticdicarboxylic acids are present in the polyester in an amount of 20–95mol %, preferably 30–90 mol %, more preferably 50–88 mol %. The unitswhich derive from the aliphatic dicarboxylic acids are present in thepolyester in an amount of 5–80 mol %, preferably 10–70 mol %, morepreferably 12–50 mol %, and the molar percentages always add up to 100%.The diol units corresponding thereto likewise always make up 100 mol %.

Preferred aliphatic dicarboxylic acids are pimelic acid, suberic acid,azelaic acid, sebacic acid, glutaric acid and adipic acid. Particularpreference is given to azelaic acid, sebacic acid and adipic acid.

Preferred aromatic dicarboxylic acids are terephthalic acid, isophthalicacid and 2,6-naphthalenedicarboxylic acid, in particular terephthalicacid and isophthalic acid.

Preferred diols are ethylene glycol, butylene glycol and neopentylglycol.

In general, the polyester comprises the following dicarboxylates andalkylenes, based in each case on the total amount of dicarboxylate ortotal amount of alkylene:

-   -   from 20 to 95 mol %, preferably from 25 to 85 mol % and more        preferably from 40 to 78 mol %, of terephthalate,    -   from 0 to 25 mol %, preferably from 5 to 20 mol % and more        preferably from 10 to 20 mol %, of isophthalate,    -   from 5 to 80 mol %, preferably from 8 to 70 mol % and more        preferably from 11 to 47 mol %, of azelate,    -   from 0 to 50 mol %, preferably from 0 to 40 mol % and more        preferably from 0.2 to 30 mol %, of sebacate,    -   from 0 to 50 mol %, preferably from 0 to 40 mol % and more        preferably from 0 to 30 mol %, of adipate,    -   more than 30 mol %, preferably more than 40 mol % and more        preferably more than 50 mol %, of ethylene or butylene.

5–40% by weight, preferably 8–35% by weight and more preferably 12–30%by weight, of the top layer material consists of a polymer which isincompatible with polyester.

Up to 10% by weight of the material of the top layer (A) consists ofparticles, additives, auxiliaries and/or other additives which arecustomarily used in polyester film technology.

It has been found to be appropriate to produce the main polyester of thetop layer (A) from two separate polyesters I and II which are fed to theextruder for this layer as a mixture.

The heatsealable and peelable top layer (A) is distinguished bycharacteristic features. It has a sealing commencement temperature(=minimum sealing temperature) with respect to the APET side ofAPET/CPET trays of not more than 150° C., preferably not more than 140°C. and more preferably not more than 130° C., and a seal seam strengthwith respect to the APET side of APET/CPET trays of at least 3.0 N,preferably at least 4.0 N, more preferably at least 4.5 N (always basedon 15 mm film breadth). The heatsealable and peelable top layer (A),with respect to the APET side of APET/CPET trays, has a maximum sealingtemperature of generally 220° C., preferably 200° C. and more preferably190° C., and a film which is peelable with respect to the APET side ofAPET/CPET trays is obtained within the entire sealing range. In otherwords, this film in the 180° tensile experiment according to FIG. 2provides a curve according to FIG. 3 b.

For the preferred, abovementioned ranges, the peeling results can alsobe described numerically. According to the present experimentalinvestigations, the peeling results can be correlated together simply bythe following relationship between the sealing temperature (∂ in ° C.)and the peeling force (in N/15 mm):0.035·∂/° C. −0.9≦peeling force F/N per 15 mm≦0.06·∂/° C.−1.4

This relationship is depicted graphically in FIG. 5 for illustration.

The film of the present invention comprises a base layer (B) and atleast one top layer (A) according to the invention. In this case, thefilm has a two-layer structure. In a preferred embodiment, the film hasa three- or more than three-layer structure. In the case of theparticularly preferred three-layer embodiment, it consists of the baselayer (B), the inventive top layer (A) and a top layer (C) on theopposite side to the top layer (A). In a four-layer embodiment, the filmcomprises an intermediate layer (D) between the base layer (B) and thetop layer (A) or (C).

The base layer of the film consists of at least 80% by weight ofthermoplastic polyester. Suitable for this purpose are polyesters ofethylene glycol and terephthalic acid (=polyethylene terephthalate,PET), of ethylene glycol and naphthalene-2,6-dicarboxylic acid(=polyethylene 2,6-naphthalate, PEN), of 1,4-bishydroxymethylcyclohexaneand terephthalic acid (=poly-1,4-cyclohexanedimethylene terephthalate,PCDT) and also of ethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalatebibenzoate, PENBB). Preference is given to polyesters which containethylene units and consist, based on the dicarboxylate units, of atleast 90 mol %, more preferably at least 95 mol %, of terephthalate or2,6-naphthalate units. The remaining monomer units stem from otherdicarboxylic acids or diols. Advantageously, copolymers or mixtures orblends of the homo- and/or copolymers mentioned can also be used for thebase layer (B). (In the specification of the amounts of the dicarboxylicacids, the total amount of all dicarboxylic acids is 100 mol %.Similarly, the total amount of all diols also adds up to 100 mol %.).

Suitable other aromatic dicarboxylic acids are preferablybenzenedicarboxylic acids, naphthalenedicarboxylic acids (for examplenaphthalene-1,4- or -1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylicacids (in particular biphenyl-4,4′-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid) or stilbene-x,x′-dicarboxylicacids. Of the cycloaliphatic dicarboxylic acids, mention should be madeof cyclohexanedicarboxylic acids (in particularcyclohexane-1,4-dicarboxylic acid). Of the aliphatic dicarboxylic acids,the (C₃–C₁₉-alkanedioic acids are particularly suitable, and the alkanemoiety may be straight-chain or branched.

Suitable other aliphatic diols are, for example, diethylene glycol,triethylene glycol, aliphatic glycols of the general formulaHO—(CH₂)_(n)—OH where n is an integer from 3 to 6 (in particularpropane-1,3-diol, butane-1,4-diol, pentane-1,5-diol and hexane-1,6-diol)or branched aliphatic glycols having up to 6 carbon atoms,cycloaliphatic, optionally heteroatom-containing diols having one ormore rings. Of the cycloaliphatic diols, mention should be made ofcyclohexanediols (in particular cyclohexane-1,4-diol). Suitable otheraromatic diols correspond, for example, to the formula HO—C₆H₄—X—C₆H₄—OHwhere X is —CH₂—, —C(CH₃)₂—, —C(CF₃—O—, —S— or —SO₂—. In addition,bisphenols of the formula HO—C₆H₄—C₆H₄—OH are also very suitable.

It is particularly advantageous when a polyester copolymer based onterephthalate and small amounts (<5 mol %) of isophthalic acid or basedon terephthalate and small amounts (<5 mol %) ofnaphthalene-2,6-dicarboxylic acid is used in the base layer (B). In thiscase, the producibility of the film and the optical properties of thefilm are particularly good. The base layer (B) then comprisessubstantially a polyester copolymer which is composed predominantly ofterephthalic acid and isophthalic acid units and/or terephthalic acidand naphthalene-2,6-dicarboxylic acid units and of ethylene glycolunits. The particularly preferred copolyesters which provide the desiredproperties of the film are those which are composed of terephthalate andisophthalate units and of ethylene glycol units.

The polyesters can be prepared by the transesterification process. Inthis process, the starting materials are dicarboxylic esters and diolswhich are reacted with the customary transesterification catalysts suchas zinc, calcium, lithium and manganese salts. The intermediates arethen polycondensed in the presence of generally customarypolycondensation catalysts such as antimony trioxide, titanium oxides oresters, or else germanium compounds. The preparation may equally well beby the direct esterification process in the presence of polycondensationcatalysts. This process starts directly from the dicarboxylic acids andthe diols.

The film of the present invention has an at least two-layer structure.It then consists of the base layer (B) and the inventive sealable andpeelable top layer (A) applied to it by coextrusion.

The sealable and peelable top layer (A) applied to the base layer (B) bycoextrusion is composed predominantly, i.e. of at least approx. 50% byweight, of polyesters.

According to the invention, the heatsealable and peelable top layer (A)comprises polyesters based on aromatic and aliphatic acids andpreferably aliphatic diols. In addition, the top layer (A) comprises apolymer which is incompatible with polyester (anti-PET polymer) in aconcentration of 5–40% by weight.

In the preferred embodiment, polyesters are copolyesters or blends ofhomo- and copolyesters or blends of different copolyesters whosecomposition is based on aromatic and aliphatic dicarboxylic acids andaliphatic diols.

Examples of the aromatic dicarboxylic acids which can be used inaccordance with the invention are terephthalic acid, isophthalic acid,phthalic acid and 2,6-naphthalenedicarboxylic acid.

Examples of the aliphatic dicarboxylic acids which can be used inaccordance with the invention are succinic acid, glutaric acid, adipicacid, pimelic acid, suberic acid, azelaic acid and sebacic acid.

Examples of the aliphatic diols which can be used in accordance with theinvention are ethylene glycol, 1,3-propanediol, 1,3-butanediol,1,4-butanediol, 1,5-pentanediol, 2,2-dimethyl-1,3-propanediol,diethylene glycol, triethylene glycol and 1,4-cyclohexanedimethanol.

The polyester for the top layer (A) is preferably prepared from twopolyesters I and II.

The proportion of the polyester I which consists of one or more aromaticdicarboxylates and one or more aliphatic alkylenes in the top layer (A)is from 10 to 60% by weight. In the preferred embodiment, the proportionof the polyester I is from 15 to 50% by weight, and in the particularlypreferred embodiment, it is from 20 to 40% by weight.

In general, the polyester I of the inventive top layer (A) is based onthe following dicarboxylates and alkylenes, based in each case on thetotal amount of dicarboxylate or total amount of alkylene:

-   -   from 70 to 100 mol %, preferably from 72 to 95 mol % and more        preferably from 74 to 93 mol %, of terephthalate,    -   from 0 to 30 mol %, preferably from 5 to 28 mol % and more        preferably from 7 to 26 mol %, of isophthalate,    -   more than 50 mol %, preferably more than 65 mol % and more        preferably more than 80 mol %, of ethylene units.

Any remaining fractions present stem from other aromatic dicarboxylicacids and other aliphatic diols, as have already been listed above asmain and secondary carboxylic acids of the base layer (B).

Very particular preference is given to those copolyesters in which theproportion of terephthalate units is from 74 to 88 mol %, thecorresponding proportion of isophthalate units is from 12 to 26 mol %(the dicarboxylate fractions adding up to 100 mol %) and the proportionof ethylene units is 100 mol %. In other words, they are polyethyleneterephthalate/isophthalate.

In a further preferred embodiment, the polyester I consists of a mixturewhich comprises a copolyester composed of terephthalate, isophthalateand of ethylene units, and an aromatic polyester homopolymer, e.g. apolybutylene terephthalate.

It has been found that in the case that the proportion of polyester I inthe top layer (A) is less than 10% by weight, the producibility of thefilm by coextrusion technology is made distinctly more difficult, or isno longer guaranteed. The tendency of the film to adhere to certainmachine parts, in particular to running metallic rolls in longitudinalstretching and after the transverse stretching, is particularly high inthis case. In contrast, when the proportion of polyester I in the toplayer (A) is on the other hand more than 60% by weight, the peelingperformance of the film is strongly impaired. The sealing performance ofthe film changes in this case from peelable to weldable.

According to the present invention, the proportion of polyester II inthe top layer (A) is from 20 to 80% by weight. In the preferredembodiment, the proportion of polyester II is from 25 to 75% by weightand in the particularly preferred embodiment, it is from 30 to 70% byweight.

The polyester II preferably consists of a copolymer of aliphatic andaromatic acid components, in which the aliphatic acid components arefrom 20 to 90 mol %, preferably from 30 to 70 mol % and more preferablyfrom 35 to 60 mol %, based on the total acid amount of the polyester II.The remaining dicarboxylate content up to 100 mol % stems from aromaticacids, preferably of terephthalic acid and/or of isophthalic acid, andalso, among the glycols, from aliphatic or cycloaliphatic or aromaticdiols, as have already been described in detail above with regard to thebase layer.

In general, the polyester II of the inventive top layer (A) is based atleast on the following dicarboxylates and alkylenes, based in each caseon the total amount of dicarboxylate or the total amount of alkylene:

-   -   from 20 to 65 mol %, preferably from 30 to 70 mol % and more        preferably from 35 to 60 mol %, of azelate,    -   from 0 to 50 mol %, preferably from 0 to 45 mol % and more        preferably from 0 to 40 mol %, of sebacate,    -   from 0 to 50 mol %, preferably from 0 to 45 mol % and more        preferably from 0 to 40 mol %, of adipate,    -   from 10 to 80 mol %, preferably from 20 to 70 mol % and more        preferably from 30 to 60 mol %, of terephthalate,    -   from 0 to 30 mol %, preferably from 3 to 25 mol % and more        preferably from 5 to 20 mol %, of isophthalate,    -   more than 30 mol %, preferably more than 40 mol % and more        preferably more than 50 mol %, of ethylene or butylene.

Any remaining fractions present stem from other aromatic dicarboxylicacids and other aliphatic diols, as have already been listed above asmain and secondary carboxylic acids for the base layer (B), or else fromhydroxycarboxylic acids such as hydroxybenzoic acid or the like.

The presence of at least 10 mol % of aromatic dicarboxylic acid ensuresthat the polymer II can be processed without adhesion, for example inthe coextruder or in the longitudinal stretching.

When the proportion of polyester II in the top layer (A) is less than20% by weight, the peeling performance of the film is strongly impaired.In this case, the sealing performance of the film changes from peelableto weldable. In contrast, when the proportion of polyester II in the toplayer (A) is on the other hand more than 80% by weight, theproducibility of the film by coextrusion technology is made moredifficult, or is no longer guaranteed. The tendency of the film toadhere to certain machine parts, in particular to running metallic rollsin longitudinal stretching and after the transverse stretching, isparticularly high in this case.

The top layer (A) preferably comprises a mixture of the polyesters I andII. Compared to the use of only one polyester with comparable componentsand comparable proportions of the components, a mixture has thefollowing advantages:

-   -   The mixture of the two polyesters I and II, from the aspect of        the particular glass transition temperatures (T_(g)s), is easier        to process (to extrude). As investigations have shown, the        mixture of a polymer having a high T_(g) (polyester I) and a        polymer having a low T_(g) (polyester II) has a lesser tendency        to adhere in the coextruder than a single polymer having a        correspondingly mixed T_(g).    -   The polymer production is simpler, because the number of        metering stations available for the starting materials generally        cannot be unlimited.    -   Moreover, from a practical aspect, the desired peeling        properties can be attained more individually with the mixture        than when a single polyester is used.    -   The addition of particles (see below) is also simpler in the        case of polyester I than in the case of polyester II.

Appropriately, the glass transition temperature of polyester I is morethan 50° C. Preference is given to the glass transition temperature ofpolyester I being more than 55° C. and more preferably more than 60° C.When the glass transition temperature of polyester I is less than 50°C., the film cannot be produced in a reliable process. The tendency ofthe top layer (A) to adhere, for example to rolls, is so high thatfrequent film breaks, in particular in the longitudinal stretching, haveto be expected. When this happens, the film can wind around the rolls inthe longitudinal stretching, which can lead to considerable damage tothe machine. In the extrusion, such a polyester adheres readily to themetallic walls and thus leads to blockages.

Appropriately, the glass transition temperature of polyester II is lessthan 20° C. The glass transition temperature is preferably less than 15°C. and more preferably less than 10° C. When the glass transitiontemperature of polyester II is greater than 20° C., the film has anincreased tendency to start to tear or tear off when pulled from thetray, which is undesired.

According to the invention, the heatsealable and peelable top layer (A)comprises a polymer which is incompatible with polyester (anti-PETpolymer) in a certain concentration. According to the present invention,the proportion of the polyester-incompatible polymer is from 5 to 40% byweight, based on the mass of the top layer (A). In a preferredembodiment, the proportion of the polymer is from 8 to 35% by weight andin the particularly preferred embodiment it is from 12 to 30% by weight,likewise based on the mass of the top layer (A).

Examples of suitable incompatible polymers are polymers based onethylene (e.g. LLDPE, HDPE), propylene (PP), cycloolefins (CO), amides(PA) or styrene (PS). In a preferred embodiment, thepolyester-incompatible polymer (anti-PET polymer) used is a copolymer.Examples thereof are copolymers based on ethylene (C2/C3, C2/C3/C4copolymers), propylene (C2/C3, C2/C3/C4 copolymers), butylene (C2/C3,C2/C3/C4 copolymers) or based on cycloolefins (norbornene/ethylene,tetracyclododecene/ethylene copolymers). In one of the particularlypreferred embodiments, the polyester-incompatible polymer is acycloolefin copolymer (COC). Such cycloolefin copolymers are describedgenerally, for example, in EP-A-1 068 949 or in JP 05-009319, which areincorporated herein by reference.

Among the cycloolefin copolymers, preference is given in particular tothose which comprise polymerized units of polycyclic olefins having anorbornene basic structure, more preferably norbornene ortetracyclododecene. Particular preference is given to cycloolefincopolymers (COC) which contain polymerized units of acyclic olefins, inparticular ethylene. Very particular preference is given tonorbornene/ethylene and tetracyclododecene/ethylene copolymers whichcontain from 5 to 80% by weight of ethylene units, preferably from 10 to60% by weight of ethylene units (based on the mass of the copolymer).

The cycloolefin polymers generally have glass transition temperaturesbetween −20 and 400° C. Suitable for the invention are those cycloolefincopolymers (COC) which have a glass transition temperature of less than160° C., preferably less than 120° C. and more preferably less than 80°C. The glass transition temperature should preferably be above 50° C.,with preference above 55° C., in particular above 60° C. The viscositynumber (decalin, 135° C., DIN 53 728) is appropriately between 0.1 and200 ml/g, preferably between 50 and 150 ml/g. Films which comprise a COChaving a glass transition temperature of less than 80° C. compared tothose comprising a COC having a glass transition temperature of greaterthan 80° C. feature improved optical properties, in particular a loweropacity.

The cycloolefin copolymers (COC) are prepared by heterogeneous orhomogeneous catalysis with organometallic compounds and is described ina multitude of documents. Suitable catalyst systems based on mixedcatalysts of titanium or vanadium compounds in combination with aluminumorganyls are described in DD 109 224, DD 237 070 and EP-A-0 156 464.

EP-A-0 283 164, EP-A-0 407 870, EP-A-0 485 893 and EP-A-0 503 422describe the preparation of cycloolefin copolymers (COC) with catalystsbased on soluble metallocene complexes. Particular preference is givento using cycloolefin copolymers prepared with catalysts which are basedon soluble metallocene complexes. Such COCs are commercially obtainable;for example Topas® (Ticona, Frankfurt).

When the proportion of the polyester-incompatible polymer is less than5% by weight, based on the weight of the top layer (A), there is nopositive influence of the polymer on the removal performance of the filmfrom the tray. When the film is removed from the tray, it still tends tostart to tear or to tear off. Especially at relatively high sealingtemperatures (>160° C.), this effect as a result of the addition ofpolyester-incompatible polymer becomes particularly apparent. Filmsproduced in accordance with the invention too then do not start to tearor tear off on removal from the tray. On the other hand, the proportionof polyester-incompatible polymer should not exceed 40% by weight, sincethe opacity of the film otherwise becomes too high.

To improve the handling of the film, the processibility of the film, butin particular also to improve the removal performance of the film fromthe tray, it is advantageous to further modify the heatsealable andpeelable top layer (A).

This is at best done with the aid of suitable antiblocking agents(particles) which are optionally added to the sealing layer and in suchamounts that the removal performance of the film from the tray isfurther improved, blocking of the film is prevented and the processingperformance of the film is optimized.

It has been found to be advantageous for at least the top layer (A) tocomprise particles in a certain size, in a certain concentration and ina certain distribution. In addition, mixtures of two and more differentparticle systems or mixtures of particle systems in the same chemicalcomposition, but different particle size, can also be added to the toplayer (A).

Customary antiblocking agents (also referred to as pigments orparticles) are inorganic and/or organic particles, for example calciumcarbonate, amorphous silica, talc, magnesium carbonate, bariumcarbonate, calcium sulfate, barium sulfate, lithium phosphate, calciumphosphate, magnesium phosphate, aluminum oxide, lithium fluoride,calcium, barium, zinc or manganese salts of the dicarboxylic acids used,carbon black, titanium dioxide, kaolin or crosslinked polystyrene oracrylate particles. The particles can be added to the layer in theparticular advantageous concentrations, for example as a glycolicdispersion during the polycondensation or via masterbatches in thecourse of the extrusion.

Particles which are preferred in accordance with the invention aresynthetic, amorphous SiO₂ particles in colloidal form. These particlesare bound into the polymer matrix in an outstanding manner and generateonly a few vacuoles (cavities). Vacuoles form at the particles in thecourse of the biaxial orientation, generally cause opacity and aretherefore little suited to the present invention. To (synthetically)produce the SiO₂ particles (also known as silica gel), sulfuric acid andsodium silicate are initially mixed together under controlled conditionsto form hydrosol. This eventually forms a hard, transparent mass whichis known as a hydrogel. After separation of the sodium sulfate formed asa by-product by a washing process, it can be dried and furtherprocessed. Control of the washing water pH and the drying conditions canbe used to vary the important physical parameters, for example porevolume, pore size and the size of the surface of the resulting silicagel. The desired particle size (for example the d₅₀ value) and thedesired particle size distribution (for example the SPAN98) are obtainedby suitable grinding of the silica gel (for example mechanically orhydromechanically). Such particles can be obtained, for example, viaGrace, Fuji, Degussa or Ineos.

It has been found to be particularly advantageous to use particleshaving an average particle diameter d₅₀ of from 2.0 to 8 μm, preferablyfrom 2.5 to 7 μm and more preferably from 3.0 to 6 μm. When particleshaving a diameter which is below 2.0 μm are used, there is no positiveinfluence of the particles on the removal performance of the film fromthe tray. In this case, the film again tends to start to tear orcontinue to tear on removal from the tray, which is of course undesired.Particles having a diameter greater than 8 μm generally cause filterproblems.

In a preferred embodiment, the diameter of the particles in theheatsealable and peelable top layer (A) is greater than the thickness ofthis layer. It has been found to be advantageous to select adiameter/layer thickness ratio of at least 1.2, preferably at least 1.3and more preferably at least 1.4. In these cases, there is aparticularly positive influence of the particles on the removalperformance of the film from the tray.

To provide the desired peeling properties, it has been found to beadvantageous when the heatsealable and peelable top layer (A) comprisesparticles in a concentration of from 0.5 to 10% by weight. Theconcentration of the particles is preferably from 0.7 to 8.0% by weightand more preferably from 1.0 to 6.0% by weight. In contrast, when thetop layer (A) of the film comprises particles and these are present in aconcentration of less than 0.5% by weight, there is no longer anypositive influence on the removal performance of the film from the tray.In contrast, when the top layer (A) of the film comprises particles andthese are present in a concentration of more than 10% by weight, theopacity of the film becomes too high.

It has been found to be particularly advantageous to use particles inthe heatsealable and peelable top layer (A) whose particle diameterdistribution has a degree of scatter which is described by a SPAN98 of≦2.0 (definition of SPAN98, see measurement method). Preference is givento a SPAN98 of ≦1.9 and particular preference to a SPAN98 of ≦1.8. Incontrast, when the top layer (A) of the film comprises particles whoseSPAN98 is greater than 2.0, the optical properties and the sealingproperties of the film deteriorate.

Moreover, it has been found to be advantageous to set the roughness ofthe heatsealable and peelable top layer (A) in such a way that its R_(a)value is greater than 60 nm. Preference is given to the roughness R_(a)being greater than 80 nm and it is more preferably greater than 100 nm;the upper limit of the roughness should not exceed 400 nm, preferably350 nm, in particular 300 nm. This can be controlled via the selectionof the particle diameters, their concentration and the variation of thelayer thickness.

In order to further improve the processing performance of the film ofthe present invention, it is advantageous likewise to incorporateparticles into the base layer (B) in the case of a two-layer filmstructure (AB), or into the nonsealable top layer (C) in the case of athree-layer film structure (ABC), in which case the following conditionsshould be observed:

-   -   The particles should have an average particle diameter d₅₀        (=median) of from 1.5 to 6 μm. It has been found to be        particularly appropriate to use particles having an average        particle diameter d₅₀ of from 2.0 to 5 μm and more preferably        from 2.5 to 4 μm.    -   The particles should have a degree of scatter which is described        by a SPAN98 of ≦2.0. Preference is given to the SPAN98 being        ≦1.9 and particular preference to the SPAN98 being ≦1.8.    -   The particles should be present in a concentration of from 0.1        to 0.5% by weight. The concentration of the particles is        preferably from 0.12 to 0.4% by weight and more preferably from        0.15 to 0.3% by weight.

To achieve the aforementioned properties, in particular the opticalproperties of the sealable and peelable film, it has been found to beappropriate, in particular in the case of a three-layer film having ABCstructure, to use a smaller amount of particles in the base layer (B)than in top layer (A). In the three-layer film of the type mentioned,the amount of particles in the base layer (B) should appropriately bebetween 0 and 2.0% by weight, preferably between 0 and 1.5% by weight,in particular between 0 and 1.0% by weight. It has been found to beparticularly appropriate only to incorporate those particles into thebase layer which get into the film via the same type of regrind(recyclate). The optical properties of the film, in particular theopacity of the film, are then particularly good.

Between the base layer and the top layers may optionally be disposedanother intermediate layer. This may in turn consist of the polymersdescribed for the base layer. In a particularly preferred embodiment,the intermediate layer consists of the polyesters used for the baselayer. The intermediate layer may also comprise the customary additivesdescribed below. The thickness of the intermediate layer is generallygreater than 0.3 μm and is preferably in the range from 0.5 to 15 μm, inparticular in the range from 1.0 to 10 μm, more preferably in the rangefrom 1.0 to 5 μm.

In the case of the two-layer and the particularly advantageousthree-layer embodiment of the film according to the invention, thethickness of the top layer (A) is in the range from 1.3 and 3.0 μm,preferably in the range from 1.4 and 2.7 μm and more preferably in therange from 1.5 and 2.4 μm. When the thickness of the top layer (A) ismore than 3.0 μm, the peeling force rises distinctly and is no longerwithin the inventive range. Moreover, the peeling performance of thefilm is impaired. In contrast, when the thickness of the top layer (A)is less than 1.3 mm, the peeling force of the film is no longer withinthe claimed range.

The thickness of the other, nonsealable top layer (C) may be the same asthe top layer (A) or different; its thickness is generally between 0.5and 5 μm.

The total thickness of the inventive polyester film may vary withincertain limits. It is from 3 to 200 μm, in particular from 4 to 150 μm,preferably from 5 to 100 μm, and the layer (B) has a proportion ofpreferably from 45 to 97% of the total thickness.

The base layer and the other layers may additionally comprise customaryadditives such as stabilizers (UV, hydrolysis), flame-retardantsubstances or fillers. They are appropriately added to the polymer orthe polymer mixture before the melting.

The present invention also provides a process for producing the film. Toprepare the inventive heatsealable and peelable top layer (A), theparticular polymers (polyester I, polyester II, polyester-incompatiblepolymer [anti-PET polymer], optionally further polymers [=for examplemasterbatch(es) for particles]) are appropriately fed directly to theextruder for the top layer (A). The materials can be extruded at fromabout 200 to 280° C. From a process engineering point of view (mixing ofthe different components), it has been found to be particularlyadvantageous when the extrusion of the polymers for the top layer (A) iscarried out using a twin-screw extruder having degassing means.

The polymers for the base layer (B) and for the further top layer (C)which may possibly be present and optionally the intermediate layer areappropriately fed to the (coextrusion) system via further extruders. Themelts are shaped to flat melt films in a multilayer die and layered ontop of one another. Subsequently, the multilayer film is drawn off withthe aid of a chill roll and optionally further rolls and solidified.

The biaxial stretching of the film is generally carried outsequentially. Simultaneous stretching of the film is also possible, butis not necessary. In the sequential stretching, preference is given tostretching first in longitudinal direction (i.e. in machine direction)and then in transverse direction (i.e. at right angles to machinedirection). The stretching in the longitudinal direction can be carriedout with the aid of two rolls rotating at different rates in accordancewith the desired stretching ratio. For transverse stretching, anappropriate tenter frame is generally used.

The temperature at which the stretching is carried out can be variedwithin a relatively wide range and depends on the desired properties ofthe film. In general, the stretching is carried out in the longitudinaldirection (machine direction orientation=MDO) in a temperature range offrom 60 to 130° C. (heating temperatures from 60 to 130° C.), and intransverse direction (transverse direction orientation=TDO) in atemperature range from 90° C. (beginning of the stretching) to 140° C.(end of the stretching). The longitudinal stretching ratio is in therange from 2.0:1 to 5.5:1, preferably from 2.3:1 to 5.0:1. Thetransverse stretching ratio is generally in the range from 2.4:1 to5.0:1, preferably from 2.6:1 to 4.5:1.

The preferred temperature range at which the biaxial stretching iscarried out in the longitudinal stretching (MDO) is from 60 to 120° C.The heating temperatures of the film in the longitudinal stretching arein the range from 60 to 115° C. In the transverse stretching (TDO), thetemperatures of the film are in the range from 90° C. (beginning of thestretching) to 140° C. (end of the stretching). The longitudinalstretching ratio in this preferred temperature range is in the rangefrom 2.0:1 to 5.0:1, preferably from 2.3:1 to 4.8:1. The transversestretching ratio is generally in the range from 2.4:1 to 5.0:1,preferably from 2.6:1 to 4.5:1.

The particularly preferred temperature range in which the biaxialstretching is carried out in the case of the longitudinal stretching(MDO) is from 60 to 110° C. The heating temperatures of the film in thelongitudinal stretching are in the range from 60 to 105° C. In thetransverse stretching (TDO), the temperatures of the film are in therange from 90° C. (beginning of the stretching) to 140° C. (end of thestretching). The longitudinal stretching ratio in this preferredtemperature range is in the range from 2.0:1 to 4.8:1, preferably from2.3:1 to 4.6:1. The transverse stretching ratio is generally in therange from 2.4:1 to 5.0:1, preferably from 2.6:1 to 4.5:1.

The preferred and especially the particularly preferred temperatures inthe MDO particularly effectively take into account the adherent behaviorof top layer (A) to rolls (metallic, ceramic or particularly coated rollsurfaces).

Before the transverse stretching, one or more surface(s) of the film canbe coated inline by the processes known per se. The inline coating maylead, for example, to improved adhesion between a metal layer or aprinting ink and the film, to an improvement in the antistaticperformance, in the processing performance or else to furtherimprovement of barrier properties of the film. The latter is obtained,for example, by applying barrier coatings such as EVOH, PVOH or thelike. In that case, preference is given to applying such layers to thenonsealable surface, for example the surface (C) of the film.

In the subsequent heat-setting, the film is kept at a temperature offrom 150 to 250° C. over a period of from about 0.1 to 10 s.Subsequently, the film is wound up in a customary manner.

The gloss of the film surface (B) in the case of a two-layer film, orthe gloss of the film surface (C) in the case of a three-layer film, isgreater than 100 (measured to DIN 67530 based on ASTM-D 523-78 and ISO2813 with angle of incidence 20°). In a preferred embodiment, the glossof these sides is more than 110 and in a particularly preferredembodiment more than 120. These film surfaces are therefore suitable inparticular for a further functional coating, for printing or formetallization.

The opacity of the film is less than 20. In a preferred embodiment, theopacity of the film is less than 15 and in a particularly preferredembodiment less than 10.

A further advantage of the invention is that the production costs of thefilm according to the invention are not substantially above those of afilm made of standard polyester. In addition, it is guaranteed that, inthe course of the production of the film, offcut material which arisesintrinsically in the operation of the film production can be reused forthe film production as regrind in an amount of up to 60% by weight,preferably from 5 to 50% by weight, based in each case on the totalweight of the film, without the physical properties of the film beingsignificantly adversely affected.

The film according to the invention is outstandingly suitable forpackaging foods and other consumable goods, in particular for packagingfoods and other consumable goods in trays in which peelable polyesterfilms are used for opening the packaging.

The table which follows (table 1) once again summarizes the mostimportant inventive film properties.

TABLE 1 Particularly Measurement Inventive range Preferred preferredUnit method Top layer (A) Proportion of units in the inventive 20 to 9530 to 90 50 to 88 mol % polyester which are based on aromaticdicarboxylic acids Proportion of units in the inventive  5 to 80 10 to70 12 to 50 mol % polyester which are based on aliphatic dicarboxylicacids Anti-PET polymer  5 to 40  8 to 35 12 to 30 % by wt. Polyester I10 to 60 15 to 50 20 to 40 % by wt. Polyester II 20 to 80 25 to 75 30 to70 % by wt. Particle diameter d₅₀ 2.0 to 8   2.5 to 7   3.0 to 6   μmFiller concentration 0.5 to 10  0.7 to 8.0 1.0 to 6.0 % Thickness of thetop layer A 1.3 to 3.0 1.4 to 2.7 1.5 to 2.4 μm Particle diameter/layerthickness ratio >/=1.2 >/=1.3 >/=1.4 Properties Thickness of the film  3to 200  4 to 150  5 to 100 μm Minimum sealing temperature of TL(A) with≦150 ≦140 ≦130 ° C. respect to APET/CPET trays Seal seam strength of TL(A) with respect  3 to 10  4 to 10 4.5 to 10  N/15 mm to APET/CPET traysGloss of the top layers A and C >70 and >100 >75 and >110 >80 and >120DIN 67530 Opacity of the film <20 <15 <10 % ASTM D 1003-52 TL: Toplayer, >/=: greater than/equal to

To characterize the raw materials and the films, the followingmeasurement methods were used for the purposes of the present invention:

Measurement of the Average Diameter d₅₀

The determination of the average diameter d₅₀ was carried out by meansof laser on a Malvern Master Sizer (Malvern Instruments Ltd., UK) bymeans of laser scanning (other measuring instruments are, for example,Horiba LA 500 or Sympathec Helos, which use the same measuringprinciple). To this end, the samples were introduced together with waterinto a cuvette and this is then placed in the measuring instrument. Thedispersion is scanned by means of a laser and the signal is used todetermine the particle size distribution by comparison with acalibration curve. The particle size distribution is characterized bytwo parameters, the median value d₅₀ (=measure of the position of theaverage value) and the degree of scatter, known as the SPAN98 (=measureof the scatter of the particle diameter). The measuring procedure isautomatic and also includes the mathematical determination of the d₅₀value. The d₅₀ value is determined by definition from the (relative)cumulative curve of the particle size distribution: the point at whichthe 50% ordinate value cuts the cumulative curve provides the desiredd₅₀ value (also known as median) on the abscissa axis.

Measurement of SPAN98

The determination of the degree of scatter, the SPAN98, was carried outwith the same measuring instrument as described above in thedetermination of the average diameter d₅₀. The SPAN98 is defined asfollows:

${SPAN98} = \frac{d_{98} - d_{10}}{d_{50}}$

The basis of the determination of d₉₈ and d₁₀ is again the (relative)cumulative curve of the particle size distribution (see above“measurement of the average diameter d₅₀”). The point at which the 98%ordinate value cuts the cumulative curve provides the desired d₉₈ valuedirectly on the abscissa axis and the point at which the 10% ordinatevalue of the cumulative curve cuts the curve provides the desired d₁₀value on the abscissa axis.

SV Value

The SV value of the polymer was determined by the measurement of therelative viscosity (η_(rel)) of a 1% solution in dichloroacetic acid inan Ubbelohde viscometer at 25° C. The SV value is defined as follows:SV=(η_(rel)−1)*1000Glass Transition Temperatures T_(g)

The glass transition temperature T_(g) was determined using film sampleswith the aid of DSC (differential scanning calorimetry). The instrumentused was a Perkin-Elmer DSC 1090. The heating rate was 20 K/min and thesample weight approx. 12 mg. In order to eliminate the thermal history,the samples were initially preheated to 300° C., kept at thistemperature for 5 minutes and then subsequently quenched with liquidnitrogen. The thermogram was used to find the temperature for the glasstransition T_(g) as the temperature at half of the step height.

Seal Seam Strength

To determine the seal seam strength, a film strip (100 mm long×15 mmwide) is placed on the APET side of an appropriate strip of theAPET/CPET tray and sealed at the set temperature of ≧140° C., a sealingtime of 0.5 s and a sealing pressure of 3 bar (sealing unit HSG/ET ofBrugger, DE, sealing jaw heated on both sides). In accordance with FIG.2, the sealed strips are clamped into the tensile testing machine (forexample Zwick, DE) and the 180° seal seam strength, i.e. the forcerequired to separate the test strips, was determined at a removal rateof 200 mm/min. The seal seam strength is quoted in N per 15 mm of filmstrip (e.g. 3 N/15 mm).

Determination of the Minimum Sealing Temperature

The Brugger HSG/ET sealing unit as described above for the measurementof the seal seam strength is used to produce heatsealed samples (sealseam 15 mm×100 mm), and the film is sealed at different temperatureswith the aid of two heated sealing jaws at a sealing pressure of 3 barand a sealing time of 0.5 s. The 180° seal seam strength was measured asfor the determination of the seal seam strength. The minimum sealingtemperature is the temperature at which a seal seam strength of at least3 N/15 mm is attained.

Roughness

The roughness R_(a) of the film was determined to DIN 4768 at a cutoffof 0.25 mm. It was not measured on a glass plate, but rather in a ring.In the ring method, the film is clamped into a ring, so that neither ofthe two surfaces touches a third surface (for example glass).

Opacity

The opacity according to Hölz was determined to ASTM-D 1003-52.

Gloss

The gloss of the film was determined to DIN 67530. The reflector valuewas measured as a characteristic optical parameter for the surface of afilm. Based on the standards ASTM-D 523-78 and ISO 2813, the angle ofincidence was set to 20°. A light beam hits the flat test surface at theangle of incidence set and is reflected or scattered by it. Thelightbeams incident on the photoelectronic detector are displayed as aproportional electrical quantity. The measurement is dimensionless andhas to be quoted together with the angle of incidence.

Tensile Strength

The tensile strength of the film was determined to DIN 53455. Theextension rate is 1%/min; 23° C.; 50% relative humidity.

Modulus of Elasticity

The modulus of elasticity of the film was determined to DIN 53457. Theextension rate is 1%/min; 23° C.; 50% relative humidity.

Shrinkage

The gloss of the film was determined to DIN 40634. The testingconditions are 150° C., 15 min.

The invention is illustrated hereinbelow with the aid of examples.

EXAMPLE 1

Chips of polyethylene terephthalate were fed to the extruder for thebase layer (B). Chips of polyethylene terephthalate and particles werelikewise fed to the extruder (twin-screw extruder) for the nonsealabletop layer (C). In accordance with the process conditions listed in thetable below, the raw materials were melted and homogenized in the tworespective extruders.

In addition, a mixture consisting of polyester I, polyester II andanti-PET polymer was prepared for the heatsealable and peelable toplayer (A). In table 2, the particular proportions of the dicarboxylicacids and glycols present in the two polyesters I and II in mol % andthe particular proportions of the components present in the mixture in %by weight are specified. The mixture was fed to the twin-screw extruderwith degassing for the sealable and peelable top layer (A). Inaccordance with the process conditions detailed in the table below, theraw materials were melted and homogenized in the twin-screw extruder.

By coextrusion in a three-layer die, the three melt streams were thenlayered on top of one another and ejected via the die lip. The resultingmelt film was cooled and a transparent, three-layer film having ABCstructure was subsequently produced in a total thickness of 25 μm by astepwise orientation in the longitudinal and transverse direction. Thethicknesses of the two top layers are each 2 μm (cf. also table 2).

Top Layer (A), Mixture of:

-   30% by weight of polyester I (=copolymer of 78 mol % of ethylene    terephthalate, 22 mol % of ethylene isophthalate) having an SV value    of 850. The glass transition temperature of polyester I is approx.    75° C. Polyester I additionally contains 5.0% by weight of SYLYSIA®    430 (synthetic SiO₂, Fuji, Japan) having a particle diameter of    d₅₀=3.4 μm and a SPAN98 of 1.7. The ratio of particle diameter d₅₀    to top layer thickness d_((A)) is 3.4:1 (cf. table 2).-   55% by weight of polyester II (=copolymer containing 40 mol % of    ethylene azelate, 50 mol % of ethylene terephthalate, 10 mol % of    ethylene isophthalate) having an SV value of 1000. The glass    transition temperature of polyester II is approx. 0° C.-   15% by weight of anti-PET polymer (=COC, TOPAS® 8007, Ticona,    Frankfurt; having a TG of approx. 75° C.)    Base Layer (B):-   100% by weight of polyethylene terephthalate having an SV value of    800    Top Layer (C), Mixture of:-   85% by weight of polyethylene terephthalate having an SV value of    800-   15% by weight of a masterbatch of 99% by weight of polyethylene    terephthalate (SV value of 800) and 1.0% by weight of SYLOBLOC® 44H    (synthetic SiO₂, Grace, Worms), d₅₀=2.5 μm, SPAN98=1.9.

The production conditions in the individual process steps were:

Extrusion Temperatures A layer: 230° C. B layer: 280° C. C layer: 280°C. Temperature of 20° C. the takeoff roll Longitudinal stretchingHeating temperature 70–100° C. Stretching temperature 105° C.Longitudinal stretching 4 ratio Transverse stretching Heatingtemperature 100° C. Stretching temperature 135° C. Transverse stretching4 ratio Setting Temperature 230° C. Time 3 s

Table 3 shows the properties of the film. According to measurements(column 2), the minimum sealing temperature of the film with respect tothe APET side of APET/CPET trays is 130° C. The film was sealed to theAPET side of APET/CPET trays at 140, 160, 180 and 200° C. (sealingpressure 4 bar, sealing time 0.5 s). Subsequently, strips of the bond ofinventive film and APET/CPET tray were pulled apart by means of atensile strain tester in accordance with the aforementioned measurementmethod (cf. FIG. 2). For all sealing temperatures, the films exhibitedthe desired peeling off from the tray according to FIG. 3 b. The sealseam strengths measured are listed in column 3. For all sealingtemperatures, peelable films were obtained. The seal seam strengths arewithin the medium to strong range, i.e. the films can be removed fromthe tray when force is applied (=easy peel). In addition, the film hadthe required good optical properties, exhibited the desired handling andthe desired processing performance.

EXAMPLE 2

In comparison to example 1, the composition of the mixture for thesealable top layer (A) was changed with otherwise identical filmstructure. The composition of the individual components remainedunchanged in comparison to example 1. The mixture now consists of thefollowing raw material proportions:

-   polyester I=25% by weight,-   polyester II=60% by weight and-   anti-PET polymer =15% by weight.

As a consequence of the higher proportion of polyester II in themixture, the process parameters were modified in the longitudinalstretching. The new conditions for longitudinal stretching are listed inthe table below:

Longitudinal stretching Heating temperature  70–95° C. Stretchingtemperature  100° C. Longitudinal stretching ratio   3.8

The minimum sealing temperature of the film with respect to the APETside of APET/CPET trays is now 122° C. For all sealing temperatures, thefilms exhibited the desired peeling off from the tray according to FIG.3 b. The seal seam strengths measured are listed in column 3. For allsealing temperatures, peelable films were again obtained. The seal seamstrengths of the films according to the invention are comparable tothose of example 1, but the seal seam strength rises less markedly withtemperature in this case. The handling and the processing performance ofthe film were as in example 1.

EXAMPLE 3

In comparison to example 1, the composition of polyester II for thesealable top layer (A) was changed with otherwise identical filmconstruction. The composition of the individual components in themixture remained unchanged in comparison to example 1. The mixture usedin top layer (A) now consists of the following raw material proportions:

-   20% by weight of polyester I, identical to example 1-   70% by weight of polyester II, VITEL® 1912, (polyester,    Bostik-Findley, USA; contains the dicarboxylic acid constituents    azelaic acid, sebacic acid, terephthalic acid, isophthalic acid and    further dicarboxylic acids approximately in the molar ratio    40/1/45/10/4, and, as the diol component, at least 60 mol % of    ethylene glycol). The glass transition temperature of polyester II    is approx. −1° C.-   10% by weight of COC (TOPAS® 8007, Ticona, Frankfurt; an    ethylene/norbornene COC having a T_(g) of approx. 75° C.)

The process parameters in the longitudinal stretching corresponded tothose in example 1. The minimum sealing temperature of the film producedin accordance with the invention to the APET side of APET/CPET trays isnow 118° C. For all sealing temperatures, the films exhibited thedesired peeling off from the tray according to FIG. 3 b. The seal seamstrengths measured are listed in column 3. For all sealing temperatures,peelable films were again obtained. The seal seam strengths of theinventive films are similar to those in example 1. The handling and theprocessing performance of the film were as in example 1.

COMPARATIVE EXAMPLE 1

In comparison to example 1, the composition of the sealable layer (A)was changed. In the top layer (A), only the polyester I based onaromatic acids was used:

-   100% by weight of polyester I (=copolymer of 78 mol % of ethylene    terephthalate and 22 mol % of ethylene isophthalate) having an SV    value of 850. The glass transition temperature of polyester I is    approx. 75° C. In addition, polyester I contains 5.0% of SYLYSIA®    430.

The production conditions in the individual process stages were adaptedin the longitudinal stretching to the glass transition temperature ofthe top layer raw material:

Longitudinal stretching Heating temperature  70–115° C. Stretchingtemperature   120° C. Longitudinal stretching ratio    4

Table 3 shows the properties of the film. Although the film is highlypigmented and the pigments constitute weak points in the sealing layer,a peelable film was not obtained for any of the specified sealingtemperatures. On removal of the film from the tray, the film started totear immediately and exhibited a diagram according to FIG. 3 a. The filmexhibited weldable behavior and is thus unsuitable for the achievementof the object specified.

COMPARATIVE EXAMPLE 2

Example 5 from EP-A-0 035 835 was reproduced. Table 3 shows theproperties of the film. A peelable film was not obtained for any of thespecified sealing temperatures. On removal of the film from the tray,the film started to tear immediately and exhibited a diagram accordingto FIG. 3 a. The film exhibited weldable behavior and is thus unsuitablefor the achievement of the object specified.

COMPARATIVE EXAMPLE 3

Example 1 from EP-A-0 379190 was reproduced. Table 3 shows theproperties of the film. A peelable film was not obtained for any of thespecified sealing temperatures. On removal of the film from the tray,the film started to tear immediately and exhibited a diagram accordingto FIG. 3 a. The film exhibited weldable behavior and is thus unsuitablefor the achievement of the object specified.

COMPARATIVE EXAMPLE 4

Example 22 from EP-A-0 379 190 was reproduced. Table 3 shows theproperties of the film. A peelable film was not obtained for any of thespecified sealing temperatures. On removal of the film from the tray,the film started to tear immediately and exhibited a diagram accordingto FIG. 3 a. The film exhibited weldable behavior and is thus unsuitablefor the achievement of the object specified.

TABLE 2 PI/PII/ Polyester I Anti- anti-PET composition Polyester IIcomposition PET- polymer TA IA EG NG AzA SeA AdA TA IA EG BD FA polymerratios mol % mol % COC % by wt Examples 1 78 22 100 40 50 10 100 10030/55/15 2 78 22 100 40 50 10 100 100 25/60/15 3 78 22 100 40  1 4510 >60 4 100 20/70/10 C Examples 1 78 22 100 — — — — — — — — — 100/0/0/2 82 18 100 — — — — — — — — — 100/0/0/ 3 — — — 10 90 100 0/100/0/ 4 100 — 84.6 15 — 32 2.4 65  1  95 4.6 — 50/50/0/ Glass transitiontemperatures PI/PII/ Top anti-PET Film layers Particles d₅₀/ polymerFilm thickness (A) (C) Diameter SPAN98 Conc d_((A)) ° C. structure μm μmμm — % ratio Examples 1 75/0/75 ABC 25 2 2 3.4 1.8 2.00 1.7 2 75/0/75ABC 25 2 2 3.4 1.8 2.00 1.7 3 75/−1/75 ABC 25 2 2 3.4 1.8 1.50 1.7 CExamples 1 75 ABC 25 1 1 3.4 1.8 5 3.4 2 75 AB 20 3 — 1.5 + 5 — 0.3 1.683 approx. 50 AB 17.2 4.1 — — — — — 4 approx. 20 AB 11.5 2.5 — 2   — 0.250.8 TA terephthalate, IA isophthalate, EG ethylene, BD butane, NGneopentyl AzA azelate, SeA sebacate, AdA adipate, FA furtherdicarboxylic acids and glycols

TABLE 3 Seal seam strength Minimum with respect to sealing APET/CPETtrays Peeling Roughnesses temperature 140° C. 160° C. 180° C. 200° C.test (=peeling Opacity Gloss A side C side ° C. N/15 mm performance) % Aside C side μm Examples 1 130 5.5 6.3 7.9 10 ++++ 15 75 130 235 60 2 1215.1 5.8 6.5 7.3 ++++ 12 89 130 256 60 3 118 5.5 6.2 6.5 7.1 ++++ 9 95130 234 60 C Examples 1 105 1.7 3.5 5 8 − 23 55 130 310 60 2 109 2 4.25.5 8.1 − 13 110 190 69 25 3 112 1.5 2 4 6 − 4 150 190 33 20 4 110 2 3 45 − 1.5 130 190 120 22 Peeling test: ++++ At all sealing temperatures,film is peeled from the tray without the film starting to tear orcontinuing to tear. Impeccable, smooth, clean peeling of the film fromthe tray, even in the upper temperature range at high seal seamstrength. − At all sealing temperatures, film starts to tear on removalfrom the tray.

1. Coextruded, transparent, biaxially oriented polyester film comprisinga base layer (B) and heatsealable top layer (A) which is peelable fromat least the APET side of an APET/CPET tray, the heatsealable andpeelable top layer (A) consisting of a) about 50–95% by weight ofpolyester and b) about 5–40% by weight of a polyester-incompatiblepolymer or anti-PET polymer, based on the mass of the top layer (A) andc) particles, wherein d) the polyester used to form said top layer (A)is composed of about 20–95 mol % of units derived from at least onearomatic dicarboxylic acid and about 5–80 mol % of units derived from atleast one aliphatic dicarboxylic acid, e) the top layer (A) exhibits aroughness, Ra, ranging from greater than 60 nm to 400 nm, as determinedvia DIN 4768, f) the layer thickness d_(A) is from about 1.3 to 3.0 μm,and g) said heatsealable top layer (A) exhibits a seal seam strengthwith respect to the APET side of APET/CPET trays of at least about 3N/15 mm.
 2. The sealable and peelable polyester film as claimed in claim1, wherein the aliphatic dicarboxylic acids are selected from one ormore of the following substances: pimelic acid, suberic acid, azelaicacid, sebacic acid, glutaric acid and adipic acid.
 3. The sealable andpeelable polyester film as claimed in claim 1, wherein the aromaticdicarboxylic acids are selected from one or more of the followingsubstances: terephthalic acid, isophthalic acid and2,6-naphthalenedicarboxylic acid.
 4. The sealable and peelable polyesterfilm as claimed in claim 1, wherein the polyester of the top layer (A)comprises: from about 20 to 95 mol % of terephthalate, from about 0 to25 mol % of isophthalate, from about 5 to 80 mol % of azelate, fromabout 0 to 50 mol % of sebacate, from about 0 to 50 mol % of adipate,more than about 30 mol % of ethylene, based in each case on the totalamount of dicarboxylate or the total amount of alkylene.
 5. The sealableand peelable polyester film as claimed in claim 1, wherein theheatsealable and peelable top layer (A) has a sealing commencementtemperature or minimum sealing temperature with respect to the APET sideof APET/CPET trays of not more than about 150° C.
 6. The sealable andpeelable polyester film as claimed in claim 1, wherein the heatsealableand peelable top layer (A) has a seal seam strength with respect to theAPET side of APET/CPET trays of at least about 4.5 N/15 mm.
 7. Thesealable and peelable polyester film as claimed in claim 1, wherein theheatsealable and peelable top layer (A) with respect to the APET side ofAPET/CPET trays has a maximum sealing temperature of about 220° C. 8.The sealable and peelable polyester film as claimed in claim 1, whereinthe sealing temperature, in ° C., and the peeling force, ∂ in N/15 mmare correlated via the following equation:0.035·∂/° C.−0.9≦peeling force F/N per 15 mm≦ 0.06·∂/° C.−1.4.
 9. Thesealable and peelable polyester film as claimed in claim 1, wherein thepolyester for the top layer (A) is produced from two polyesters I andII.
 10. The sealable and peelable polyester film as claimed in claim 9,wherein the proportion of the polyester I in the top layer (A) is fromabout 10 to 60% by weight.
 11. The sealable and peelable polyester filmas claimed in claim 10, wherein the polyester I consists of one or morearomatic dicarboxylates and one or more aliphatic alkylenes.
 12. Thesealable and peelable polyester film as claimed in claim 9, wherein theproportion of polyester II in the top layer (A) is from about 20 to 80%by weight.
 13. The sealable and peelable polyester film as claimed inclaim 12, wherein the polyester II consists of one or more aromaticdicarboxylates and also one or more aliphatic dicarboxylates and one ormore aliphatic alkylenes.
 14. The sealable and peelable polyester filmas claimed in claim 9, wherein the glass transition temperature ofpolyester I is more than about 50° C.
 15. The sealable and peelablepolyester film as claimed in claim 9, wherein the glass transitiontemperature of polyester II is less than about 20° C.
 16. The sealableand peelable polyester film as claimed in claim 1, wherein the particleshave an average particle diameter ranging from 2.0 to 8 μm and aparticle diameter distribution with a degree of scatter which isdescribed by a SPAN98 of ≦ about 2.0.
 17. The sealable and peelablepolyester film as claimed in claim 1, wherein the film has two layersand an AB structure.
 18. The sealable and peelable polyester film asclaimed in claim 1, wherein the film has three layers and an ABCstructure.
 19. A process for producing a sealable and peelable polyesterfilm as claimed in claim 1, in which the polymers forte base layer (B)and the top layer (A) and, optionally, the top layer (C) are fed toseparate extruders, the melts are then shaped and layered on top of oneanother in a multilayer die to give flat melt filing, then to multilayerfilm is drawn off with the aid of a chill roll and optionally furtherrolls, solidified and then biaxially stretch-oriented and heat-set, thebiaxial stretching being carried out in succession, first longitudinallyin machine direction and then transversely at right angles to machinedirection wherein the longitudinal stretching is carried out at atemperature in the range from about 60 to 130° C. and the transversestretching in the range from about 90 to 140° C., and the longitudinalstretching ratio is set within the range from about 2.0:1 to 5.5:1 andthe transverse stretching ratio is within the range from about 2.4:1 to5.0:1.
 20. The process as claimed in claim 19, in which the longitudinalstretching is carried out at a temperature in the range from about 60 to120° C. and the transverse stretching in the range from about 90 to 140°C. and the longitudinal stretching ratio is in the range from about2.0:1 to 5.0:1 and the transverse stretching is in the range from about2.4:1 to 5.0:1.
 21. The process as claimed in claim 19, in which thelongitudinal stretching is carded out at a temperature in the range fromabout 60 to 110° C. and the transverse stretching in the range fromabout 90 to 140° C. and the longitudinal stretching ratio is set withinthe range from about 2.0:1 to 4.8:1 and the transverse stretching is inthe range from about 2.4:1 to 5.0:1.
 22. A lid film for coveringAPET/CPET trays comprising the sealable and peelable polyester film asclaimed in claim
 1. 23. Coextruded, transparent, biaxially orientedpolyester film comprising a base layer (B) and heatsealable top layer(A) which is peelable from at least the APET side of an APET/CPET tray,the heatsealable and peelable top layer (A) consisting of a) about 50 to95% by weight of polyester and b) about 5 to 40% by weight of apolyester-incompatible polymer or anti-PET polymer, based on the mass ofthe top layer (A), wherein c) the polyester used to form said top layer(A) is composed of about 20 to 95 mol % of units derived from at leastone aromatic dicarboxylic acid and about 5 to 80 mol % of units derivedfrom at least one aliphatic dicarboxylic acid, d) the layer thicknessd_(A) is from about 1.3 to 3.0 μm, and e) the anti-PET polymer isselected from one or more of the following substances: polymers based onethylene, propylene, cycloolefins, amides and styrene.
 24. The sealableand peelable polyester film as claimed in claim 23, wherein the anti-PETpolymer is selected from one or more of the following substances:copolymers based on norbornene/ethylene and tetracyclododecene/ethylene.